Just as people can tell if you are confident by meeting you, they can also tell if you are confident from looking at your signature. Often without any handwriting analysis training, too!
But the nice thing about confidence is that it is one of the “fake it till you make it” traits. You can make yourself feel and look, and write, with confidence at least over the short span, even if you don’t feel very confident most of the time.
Let’s say you are going to put your signature to an important document and you want to show confidence. It could be the cover letter as you apply for that all important job.
Get the letter all typed, printed out and ready for your signature.
Now sit back, relax, close you eyes, and think confidence. Tell yourself how confident you are. Visual the results of you being confident, of your confidence being rewarded (in this scenario it would be getting the job). Keep visualizing and thinking confidence, and when you feel you have achieved it, open your eyes and write your name, without taking time to allow your doubts to return. Write your name thinking how proud you are to be you, to be the possessor of all the great traits and abilities that you have.
This should make your signature more confident in general.
One of the ways confidence shows in a signature is when it is slightly larger than the rest of the writing – but not too big, that would indicate arrogance.
So sign with confidence. And then having seen what difference there is in your confident signature over previous versions, keep doing the confident one from now on. Soon it will be part of you.